1. Field of the Invention
The present invention relates to an ultrasonic cavitating apparatus for causing cavitation in a fluid flowing through a pipe, and an ultrasonic Doppler flow measurement system employing the ultrasonic cavitating apparatus.
2. Description of the Related Art
A generally used ultrasonic flow measuring method of measuring the flow rate of a fluid flowing through a pipe uses, for example, propagation time difference method as mentioned in “Cho-Ompa Gijutu Binran (p. 746)”, Saneyoshi et al., The Nikkan Kogyo Shimbun Ltd. However, when the inner surface of the pipe has a high surface roughness or when the inner surface of the pipe is covered with a cladding of, for example, rust, the accuracy of measurement by the above method lowers depending on the sectional area of a part having a high surface roughness or the cladding.
A measurement system developed to solve such a problem and applied to practical use is designed to improve accuracy by multiplying a measured flow velocity by a profile factor. Such a system is mentioned in “Calibration for Beaver Valley Unit 2 LEFM CheckPlus System® (2000) published in the Web site of Caldon Inc., USA. However, since the profile factor changes when the thickness of the cladding changes in a long time of use, measuring accuracy lowers accordingly.
To solve such a problem, flow measurement systems that measure the flow rate of a fluid based on the Doppler shift of ultrasonic wave have been proposed. Such systems are mentioned in JP6-294670A, and “Development of Flow Rate Measurements Using Ultrasonic Velocity Profile Method (11)”, Otsukasa et al., Preliminary Papers, H13, for 2002 Annual Meeting (Spring) of Atomic Energy Society of Japan (Mar. 27-29, 2002).
The flow measuring method using Doppler shift analysis is based on the fact that the frequency of an echo reflected from minute ultrasonic reflectors existing in a fluid varies depending on the flow velocity of the fluid, which fact can be expressed by the following well-known Expression (1).
                    f        =                                            c              -              v                                      c              +              v                                ⁢                      f            0                                              (        1        )            where f is the frequency of the reflected echo, v is the flow velocity of the fluid, c is sound velocity (m/s) in the fluid, and f0 (Hz) is the frequency of the ultrasonic wave emitted toward the minute reflectors.
The ultrasonic Doppler flow measurement system is capable of measuring the distribution of flow velocity of a fluid and does not use the profile factor which is used by the propagation time difference method. Therefore, the ultrasonic Doppler flow measurement system is able to achieve accurate flow measurement without being affected by the surface roughness of the inner surface of a pipe or cladding coating the inner surface of a pipe.
An ultrasonic Doppler flow measurement system disclosed in JP6-294670A generates an ultrasonic wave in a pipe by an ultrasonic radiator to cause cavitation, and generates ultrasonic pulses in the pipe by an ultrasonic transducer attached obliquely to the pipe. The ultrasonic pulse is reflected by bubbles produced by cavitation. The frequency of the reflected pulses is compared with that of the generated ultrasonic pulses. The flow velocity v of the fluid can be calculated by using Expression (1).
Sound velocity in a liquid is dependent on the temperature of the liquid. Therefore, condition for cavitation is dependent on the temperature of the liquid. The ultrasonic Doppler flow measurement system disclosed in JP6-294670A does not have any function to adjust parameters, such as the frequency of the ultrasonic wave, in response to the change in the liquid temperature. Therefore, cavitation caused at a temperature disappears when the temperature changes.